What is an error budget and how is it used in SRE?

An error budget is the allowable amount of unreliability a service can have within a given SLO window. If your SLO promises 99.9% availability over 30 days, you have 0.1% of that window to spend on failures — roughly 43.2 minutes of downtime. That 43.2 minutes is your error budget.

The budget is consumed whenever the SLI falls below the SLO target. Consumption is tracked in real time. When the budget is healthy (plenty remaining), teams have license to deploy frequently and take calculated risks. When the budget is nearly exhausted, deployments freeze until reliability recovers — this is the error-budget policy.

Error budgets eliminate the adversarial relationship between development velocity and reliability. Developers are incentivized to invest in reliability work because burning the budget costs them deployment freedom. SREs can quantify risk without saying "no" to every release: instead, the budget says how much risk is left.

The burn rate concept extends this further. A burn rate of 1 means you are consuming the budget exactly in line with the window. A burn rate of 10 means you will exhaust the budget ten times faster than the SLO window allows — a signal to page on-call immediately rather than wait for a daily report.